Beginner’s Guide: Exposure

If you’re just getting to grips with the fundamentals of photography it may seem like there is a lot of information to take in. An introductory book can be helpful here, but when I was starting out I could have really used a quick reference of the essential facts that I needed to commit to memory that did not confuse me lots of nuanced discussion of the key topics. This will be the first in a series of posts in which I will attempt to provide that quick reference for beginners. This post will deal with the most important topic in all of photography: exposure.

Exposure: a definition

Exposure is the defined as the amount of light reaching the camera’s sensor. A correct exposure is neither under-exposed (too dark) or over-exposed (too light). Your objective above all other concerns is to allow the right amount of light to hit the camera’s sensor to get the correct exposure.

Controlling exposure

Cameras take photos by recording the amount of light that comes through a hole in the lens (called the aperture) over a period of time (shutter speed) at a defined sensitivity to light (ISO value). You can change these three parameters to affect the amount of light reaching the camera’s sensor, and thus your exposure. In most camera modes except manual, the camera will chose the values of one or two of these parameters automatically. For example, in Aperture Priority mode, you set the aperture and the camera works out the shutter speed (and ISO, if you’re using AUTO ISO) to give the correct exposure.

It is crucial to recognise that when you change one of these three parameters to let in more light, you must make an equal and opposite change to one of the other two parameters to ensure the same amount of light is captured. For example, if you have a correct exposure and you decide you need to make the shutter speed half as long, that your exposure will capture half as much light so you must either make the aperture twice as large or the ISO sensitivity twice as sensitive to balance things out.

A helpful analogy is to imagine filling a bucket of water from a tap. There are three factors that will determine how much water ends up in the bucket: the diameter of the pipe (aperture), how long you open the tap for (shutter speed) and the water pressure (ISO value). Logically it should be obvious that you can get the same amount of water into the bucket with a pipe diameter of twice the size in half the time, or a pipe the same size in half the time with twice the water pressure.

Choosing exposure parameters

As we have said, you can change the amount of light reaching the camera’s sensor by changing the aperture, the shutter speed or the ISO value. So why would you choose one of these settings over the other?

Aperture affects depth of field. A wider aperture (a larger hole) causes less of the image to be in focus and makes the background more blurry. This is usually a good thing for portraits where you want to isolate your subject and blur the background, and a bad thing for landscapes where you want the whole scene in focus.

Slow shutter speeds create motion blur effects. This is useful when photographic running water or night scenes (on a tripod) or (with more difficulty) moving vehicles. Most of the time though it just ruins the photo because you can’t hold the camera still for long enough.

Higher ISO values create noise in the exposure. Noise makes the photo look grainy or have coloured dots in it, especially when you zoom to view the photo full size. It happens because with higher ISO values the camera amplifies the light signal and when you take a very small light reading and multiply it several times there is a greater chance of inaccuracy in the value of light recorded for each pixel. There is no positive benefit from using higher ISO values – you should always use the lowest value you can get away with. You only increase ISO because you want to use a particular combination of aperture and shutter speed, and the ISO value is the only parameter left that you can change to get a correct exposure.

What values should I choose?

Aperture is measured in numbers called f-stops, writtenwith a notation like f/2 or f/5.6. It’s a ratio between the size of the hole in the lens and the focal length of the lens, but don’t worry about that for now; what you need to remember is that smaller numbers on the f-stop range represent larger apertures. This is confusing for most people at first. Your lens will have a maximum (and minimum) aperture, which will typically be printed on the front or side. The largest (or as it is more commonly described, widest) aperture will usually be somewhere between f/1.4 and f/5.6. Many photographers recommend that you don’t use lenses at their absolute widest aperture because the lens will take sharper photos when stopped down slightly to a smaller aperture. Personally I’ve found this effect to be negligible on my lenses but I recommend doing your own experiments with this. Essentially, you want a wide aperture (low f-stop number) for portraits and things, and a smaller aperture for landscapes or situations where you want the background to be in focus.

Shutter speed is measured in seconds or fractions of a second. If you’re using a tripod to shoot a landscape shot, you’ll probably want a long shutter speed of anywhere from 5 – 60 seconds or more. For most other situations you’ll want the fastest shutter speed possible, definitely fast enough to avoid blur from camera shake if your camera isn’t in a fixed position. There’s a useful rule to calculate a safe shutter speed to avoid camera shake: use a shutter speed of at least 1 divided by the focal length of the lens. In other words, if you are using a 70-200mm lens zoomed to 200mm and hand held, you need a shutter speed of at least 1/200 sec or faster.

ISO is measured as an integer, with 100 generally considered to be normal. Any higher value is considered to be amplifying the light signal. As far as I’m aware the number 100 here doesn’t mean anything in and of itself – it’s just an arbitrary baseline notation of what normal looks like. ISO values are simple to work with; if you use ISO 200 then your sensor will be twice as sensitive to light as ISO 100, and at ISO 400 it will be twice as sensitive again. You should always try to use the lowest ISO value possible (on my camera that is ISO 100 – on Nikon SLRs it is usually ISO 200 and I’ve seen cameras that have an ISO 50 setting), but do not use a low ISO value if doing so means using a shutter speed that will cause camera shake. Most cameras perform pretty well up to ISO 800, and top end SLRs perform well at ISO 1600 in most conditions. You can correct a certain amount of noise from high ISO in post-production but you cannot recover from camera shake.

Choosing a camera mode

In full manual mode (which almost all cameras have) you set the aperture, shutter speed and ISO value yourself, using a hand-held light meter, the light meter built into the camera itself or good old-fashioned experience to judge when you have got the combination right. I don’t use full manual mode much, but photographers who do will usually tell you that it gives better consistency across a series of photos because the camera isn’t constantly changing parameters shot by shot, and that it gives greater control, as the camera’s metering system for judging exposure can be fooled into over- or under-exposing a shot when the subject is very bright or very dark. You almost certainly don’t want to use this mode when you’re starting out because you will find yourself overwhelmed trying to focus on all the camera settings as well as your composition. So which mode should you use?

If your camera has an Auto mode, the only time you should even consider using it is if you’re in a situation where you just need to capture a moment and you don’t have the time or the confidence to guarantee results with a mode that offers more control. With Auto mode your results will always be hit and miss because you are controlling nothing except composition. You will be a slave to the whims of your camera at any given moment. Pro cameras do not even have this mode because no self respecting pro would ever use it.

Most cameras have a Program mode which is similar to full auto mode except you can control the ISO value, exposure compensation and flash. I find no use for it for the same reasons as with Auto mode: you almost always want to control either the aperture or the shutter speed as they influence composition, and with Program mode you can change neither.

Consumer grade SLR and point-and-shoot cameras have “scene” modes (candle light, night portrait, landscape, macro etc). These modes hint to the camera what kind of subject you’re trying to photograph and cause the camera to use a variant of Auto mode that is tuned to pick exposure parameters conducive to that subject (for example the portrait modes will use wide apertures to create shallow depth of field). These methods have their place when you’re starting out and most of the time you will get results with them, but try not to over-rely on these settings; in doing so you’re not really learning how exposure works, and if you move up to a prosumer or pro camera model which doesn’t have these modes you’ll be lost.

Aperture priority mode lets you specify the aperture (and optionally, ISO value) and the camera chooses the shutter speed to match. This is the mode I use most, because for most compositions I care about what depth of field I get (and when I’m not using a tripod I’m always going to want the shortest shutter speed possible for my selected aperture).

Shutter priority mode lets you specify the shutter speed (and optionally, ISO value) and the camera chooses the aperture. I use this mode when I’m working with a tripod to take a long exposure of running water or a night scene where I want the clouds to streak across the sky as the move. Another scenario you might want to use this mode is where you’re photographing fast motion (such as sports or a bird in flight) and you need to lock in a fast shutter speed.

Helpful hints

If your camera supports shooting in the RAW format, use it. This file format stores the raw values of light intensity and colour from the sensor rather than creating a compressed JPEG image in-camera. There are many advantages to this approach, but the most important is that the RAW format gives you significantly greater latitude to recover an under- or over-exposed shot in software because it retains more detail than a JPEG.

Different exposure settings are useful in different scenarios, so try not to get fixated on a single way of doing things. Experiment, and read up on recommended settings for a given photographic subject if you aren’t sure.

If your camera has a built-in flash, turn it off when using your camera creatively. You may need it occasionally to take a purely functional photo, but built-in flashes will always produce a very hard light with harsh shadows that are unpleasant to the eye. When you’re ready, buy a proper speedlight if you want to experiment with flash.